Depending on the age of the "old pipe" the carbon equivalent (CEIIW) can be from about 0.3 to 0.5 so it helps to know what you have. If we were doing this we would probably use SMAW with E6010 root+E8010 hot, fill, cap with a full penetration weld, unless the specific design allowed a fillet weld. We would normally not preheat a 0.375 wall pipeline unless we thought we had one of the very high CE versions, but we would often preheat a plate 1" thick to about 250-300F to be on the safe side. Note that this is from the viewpoint of gas pipeline welding. The structural side of the discussion board readership might recommend something different.